Since the advent of sound recording near the end of the nineteenth century, ways have been sought to make the reproduction of sound, and especially music, approach as closely as possible the sound field created by an original source of sound. Despite occasional announcements that the ultimate of perfection has been reached, in the intervening time up until the present, much experimentation and theorizing has continued with the object in mind of so improving the quality of sound reproduction that the listener is persuaded he is hearing a live performance.
In earlier times, attempts to achieve this ideal centered around a series of efforts to improve the linearity of the various transducers, amplifiers, broadcast and receiving means in the audio chain. Although these improvements were of great benefit in enhancing the quality of reproduced sound, it became apparent that no amount of improvement in the linearity of reproduction by itself could adequately recreate the auditory experience of listening to a live source of music. This was especially true in the reproduction of music produced by such large groups as bands, orchestras and choruses, but was present to a lesser degree also in the sound from solo instrumentalists and vocalists.
Consequently, since approximately the middle 1950's growing emphasis has been placed upon attempts to record and reproduce the phase and aural space relationships of live sound sources. Specifically, it was realized that the abilities of the human ear to spatially locate sound sources based upon cues derived from phase relationships and relative intensity differences between sound heard by the left and right ear was considerable. Consequently, attempts to more adequately account for these sensitivities of human hearing in the design of audio systems brought about a considerable interest in multiple-channel recording and reproduction of sound, in phase relationships among the several drivers in loudspeaker systems, and in the accurate recreation of the reflected or ambient sound field which naturally results from the production of sound in an enclosed space such as a room or hall, due to the considerable reflection of sound from the walls, ceiling and floor.
An accurate portrayal of the reflected sound field is believed to be particularly important not only because reflected sound comprises a significant part of the sound actually heard by a listener at a symphonic concert, for example, but also because reflected sound, travelling as it does a longer path from source to listener and approaching him from a different angle than the original source of sound, considerably alters the harmonic content of perceived sounds and the perceived size and location of the sound source itself.
In order to effectively duplicate the richness and complexity generated by a significant reflected sound field, the loudspeaker must possess to a considerable degree the quality known as dispersion by which is meant the ability to distribute reproduced sound over a very wide angle. In this way, reflected sound from all of the reflective sounding surfaces of a room, for example, can be generated such that the perception of the sound field approximates that of an original source of sound.
Individual electrodynamic drivers in which the diaphragm is a cone or hemisphere are capable of providing near-perfect dispersion only for frequencies low enough such that the reproduced wavelength is large in comparison to the effective piston diameter of the driver. For our purposes, the effective piston diameter may be defined as the diameter of a circular diaphragm which, driven to the same mean excursion, would generate the same sound pressure level (SPL) as the actual driver under consideration. For many cone-diaphragm loudspeakers, effective piston diameter in the frequency range below cone "break-up" may be approximated as 0.9 times nominal loudspeaker diameter. Since the upper frequency limit of audible sound extends at least as high as 15 kHz, where the wavelength is only 2.3 cm., and down to as low as 30 Hz where the wavelength is approximately 11.5 m., it becomes obvious that maintaining equally broad dispersion at all frequencies in the audible spectrum is a difficult task!
An ideal radiator for achieving this goal might be conceived in the form of a point source of sound covering equally well the entire audible range of 20-20,000 Hz. However, a few simple calculations reveal the impracticality of ever actualizing such a source in practice. The volume of air required to be moved, or "pumped", in order to produce a peak sound pressure level of 110-120 db (SPL) such as would be required to adequately reproduce sharp attacks during fortissimo passages of orchestral music would require that the theoretical point source of sound be replaced by a pulsating sphere of considerable dimensions.
In fact such a sphere can, using existing technology, only be approximated by mounting a plurality of discrete drivers spaced over the surface of a spherical enclosure. Such reproducers have been built from time to time (see for example U.S. Pat. No. 4,006,308 to Ponsgen which utilizes a hemispherical enclosure).
Unfortunately, when this approach is extended to a full sphere, and when reasonable driver efficiency levels are considered and reasonable sound pressure levels such as 110 db are contemplated at frequencies below 50 Hz, the required spherical enclosure will be uncomfortably large and the number of drivers which must be spaced over its surface in order to provide adequate dispersion at all frequencies becomes so large that the approach cannot be considered practicable either from the standpoint of aesthetics or economics.
For example, it has been calculated that a practical design capable of producing a sound pressure level of 110 db at 20 Hz and having efficiency of approximately 90 db (SPL) at one meter for a one watt input would require a spherical enclosure having an internal volume of approximately seven cubic feet, or a diameter of nearly three feet! Since such a spherical loudspeaker would have extremely limited acceptance in the high fidelity marketplace, some way was needed to reduce its size while retaining its abilities to simulate the sonic characteristics of a wide frequency range point source of sound.
As is obvious to those skilled in the art, the requirement for such a large sphere results virtually entirely from the requirement for large products of diaphragm movement and diaphragm area in order to generate high amplitudes of sound at low bass frequencies.
Consequently, the principal line of approach to reduction in the size of the sphere might begin with a division of the loudspeaker system into a lower frequency reproducer located outside the sphere with the higher frequency reproducers remaining on the surface of the sphere, now much reduced in size.
While such division of the frequency spectrum into two parts is entirely routine, requiring only the use of either active or passive filter networks, it was realized that if the system were to function as an accurate simulator of a wide frequency range point source of sound, provision had to be made to cause at least the upper portion of the frequency range from the lower frequency reproducer to appear to emanate from the sphere in coincidence with the sound from the higher frequency reproducers located on the sphere surface.
Since the human ear has little ability to localize the source of extremely low frequency acoustic energy (say, below 90 Hz or thereabouts), no particular provision need be made for enhancing the already excellent dispersion of this band of frequencies. However, if the lower frequency reproducer is to be called upon to extend much above the frequency range of the lowest bass notes, it is important to make its output in this range appear to emanate from the center of the sphere. Moreover, it is important that the dispersion of the upper portion of the frequency range of the lower frequency reproducer be smooth and uniform, especially in the horizontal plane. Finally, it is important that the path length from the low frequency reproducer to listeners in the far field of the loudspeaker be sufficiently identical to the path length from the higher frequency reproducers such that phase coherency problems are minimized or at least small enough to be easily corrected.